Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2001-10-29
2004-09-07
Jackson, Jerome (Department: 2815)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S038000, C349S042000, C349S045000
Reexamination Certificate
active
06788357
ABSTRACT:
This application claims the benefit of Korean Patent Application No. 2000-64379, filed on Oct. 31, 2000 in Korea, which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an array substrate for a liquid crystal display device.
2. Description of Related Art
Generally, liquid crystal display (LCD) devices make use of optical anisotropy and polarization properties of liquid crystal molecules to control alignment orientation. The alignment direction of the liquid crystal molecules can be controlled by application of an electric field. Accordingly, when the electric field is applied to liquid crystal molecules, the alignment of the liquid crystal molecules changes. Since refraction of incident light is determined by the alignment of the liquid crystal molecules, display of image data can be controlled by changing the applied electric field.
Of the different types of known LCDs, active matrix LCDs (AM-LCDs), which have thin film transistors and pixel electrodes arranged in a matrix form, are of particular interest because of their high resolution and superiority in displaying moving images. Because of their light weight, thin profile, and low power consumption characteristics, LCD devices have wide application in office automation (OA) equipment and video units. A typical liquid crystal display (LCD) panel may include an upper substrate, a lower substrate and a liquid crystal layer interposed therebetween. The upper substrate, commonly referred to as a color filter substrate, may include a common electrode and color filters. The lower substrate, commonly referred to as an array substrate, may include switching elements, such as thin film transistors (TFTs), and pixel electrodes.
LCD device operation is based on the principle that the alignment direction of the liquid crystal molecules is dependent upon an electric field applied between the common electrode and the pixel electrode. Moreover, because the liquid crystal molecules have spontaneous polarization characteristics, the liquid crystal layer is considered an optical anisotropy material. As a result of the spontaneous polarization characteristics, the liquid crystal molecules possess dipole moments when a voltage is applied to the liquid crystal layer between the common electrode and pixel electrode. Thus, the alignment direction of the liquid crystal molecules is controlled by the application of an electric field to the liquid crystal layer. When the alignment direction of the liquid crystal molecules is properly adjusted, incident light is refracted along the alignment direction to display image data. The liquid crystal molecules function as an optical modulation element having variable optical characteristics that depend upon polarity of the applied voltage.
FIG. 1
shows a conventional LCD device. The LCD device
11
includes an upper substrate
5
and a lower substrate
22
with a liquid crystal layer
14
interposed therebetween. The upper substrate
5
and the lower substrate
22
are commonly referred to as a color filter substrate and an array substrate, respectively. Within the upper substrate
5
and upon the surface opposing the lower substrate
22
, a black matrix
6
and a color filter layer
7
are formed in the shape of an array matrix and include a plurality of red (R), green (G), and blue (B) color filters so that each color filter is surrounded by corresponding portions of the black matrix
6
. Additionally, a common electrode
18
is formed on the upper substrate
5
to cover the color filter layer
7
and the black matrix
6
. In the lower substrate
22
and upon the surface opposing the upper substrate
5
, a thin film transistor (TFT) “T,” is formed in the shape of an array matrix corresponding to the color filter layer
7
. A plurality of crossing gate lines
13
and data lines
15
are positioned such that each TFT “T” is located adjacent to each crossover point of the gate lines
13
and the data lines
15
. Furthermore, a plurality of pixel electrodes
17
are formed on a pixel region “P” defined by the gate lines
13
and the data lines
15
of the lower substrate
22
. The pixel electrode
17
includes a transparent conductive material having good transmissivity such as indium-tin-oxide (ITO) or indium-zinc-oxide (IZO), for example.
According to the LCD device
11
of
FIG. 1
, a scanning signal is applied to a gate electrode of the TFT “T” through the gate line
13
, while a data signal is applied to a source electrode of the TFT “T” through the data line
15
. As a result, the liquid crystal molecules of the liquid crystal layer
14
are aligned and arranged by operation of the TFT “T,” and incident light passing through the liquid crystal layer
14
is controlled to display an image.
FIG. 2A
is a plan view showing a pixel of a conventional array substrate for use in a liquid crystal display device. In
FIG. 2
, an array substrate
22
includes a pixel region “P” having a corresponding thin film transistor (TFT) “T,” a pixel electrode
17
and a storage capacitor “C.” Gate lines
13
are arranged in a transverse direction and data lines
15
are arranged in a longitudinal direction such that each pair of the gate lines
13
and the data lines
15
define a pixel region “P.” The TFT “T” includes a gate electrode
26
, a source electrode
28
, a drain electrode
30
and an active layer
33
. The gate electrode
26
of the TFT “T” extends from the gate line
13
, while the source electrode
28
of the TFT “T” extends from the data line
15
. The drain electrode
30
is spaced apart from the source electrode
28
and the active layer
55
is disposed over the gate electrode
24
between the source electrode
28
and the drain electrode
30
. The source electrode
28
and the drain electrode
30
overlap opposite ends of the gate electrode
26
. A portion of the pixel electrode
17
overlaps a portion of the drain electrode
30
and electrically contacts the drain electrode
30
through a drain contact hole
41
. Furthermore, the storage capacitor “C” is a storage-on-gate type capacitor, and thus comprises a capacitor electrode
16
electrically communicating with a pixel electrode
17
through a capacitor contact hole
43
, a portion of the gate line
13
, and an insulator functioning as a dielectric layer (not show in FIG.
2
). Namely, the storage capacitor “C” has M/I/M (metal/insulator/metal) structure. At this point, the position and configuration of the storage capacitor “C” can be various.
In the above-described structure, a parasitic capacitor is formed between the gate electrode
26
and the drain electrode
28
of the TFT “T.” The parasitic capacitance influences and deteriorates function of the liquid crystal layer since the parasitic capacitance is a direct-current component of the voltage. Furthermore, the gate electrode
26
and the drain electrode
28
of the TFT “T” can become short-circuited if the gate insulation layer disposed on the gate electrode
26
has defects, such as pinholes or cracks. Accordingly, the gate insulation layer in a conventional array substrate, which is used as a dielectric layer in the storage capacitor “C,” is formed of a relatively large thickness over the gate electrodes and gate lines.
Thin film transistors (TFTs) can be divided into two generally different categories based upon the relative disposition of their gate electrodes—staggered types and coplanar types. The staggered type TFT includes an inverted staggered type which is generally used for LCD devices due to their simple structure and superior efficiency. Within the inverted staggered type TFT there includes a back channel etched type (EB) and an etch stopper type (ES). A manufacturing method of the back channel etched type TFT will be explained hereinafter.
FIGS. 3A
to
3
J are plan views and cross-sectional views each taken along the line III—III of corresponding plan view and illustrates conventional manufacturing processes of an array substrate of FIG.
2
B.
Re
Jackson Jerome
LG.Philips LCD Co. , Ltd.
Morgan & Lewis & Bockius, LLP
Nguyen Joseph
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